The present invention relates to a charging device having a charging member for charging a member to be charged such as a photosensitive member or a dielectric member, and an image forming apparatus having the same.
Heretofore, a corona type charger (corona discharging device) has been widely used as a charging apparatus for charging (inclusive of discharging) an image bearing member (member to be charged) such as an electrophotographic photosensitive member or an electrostatic dielectric recording member to a predetermined polarity and a predetermined potential level in an image forming apparatus, for example, an electrophotographic apparatus or an electrostatic recording apparatus.
The corona type charging device is a non-contact type charging device, and comprises a corona discharging electrode such as a wire electrode, and a shield electrode which surrounds the corona discharging electrode. It is disposed so that corona discharging opening thereof faces an image bearing member, that is, a member to be charged. In usage, the surface of an image bearing member is charged to a predetermined potential level by being exposed to discharge current (corona shower) generated as a high voltage is applied between the corona discharging electrode and the shield electrode.
Recently, a charging device of a contact type (contact charging device) is put into practice wherein the member to be charged is charged by contacting the charging member supplied with a voltage to the member to be charged in view of its advantage of lower production of ozone and lower required electric power, as compared with the corona charger.
In order to charge a member such as an image bearing member with the use of a contact type charging apparatus, the electrically conductive charging member (contact type charging member, contact type charging device, or the like) of a contact type apparatus is placed in contact with the member to be charged, and an electrical bias (charge bias) of a predetermined level is applied to this contact type charging member so that surface of the member to be charged is charged to a predetermined polarity and a predetermined potential level. The charging member is available in various forms, for example, a roller type (charge roller), a fur brush type, a magnetic brush type, a blade type, and the like.
When a member is electrically charged by a contact type charging member, two types of charging mechanisms (charging mechanism or charging principle:
(1) mechanism which discharges electrical charge, and
(2) mechanism for injecting charge) come into action. Thus, the characteristics of each of contact type charging apparatuses or methods are determined by the charging mechanism which is the dominant one of the two in charging the member. FIG. 11 shows a typical charging properties of them.
(1) Electrical Discharge Based Charging Mechanism
With this mechanism, the surface of the member to be charged is charged by the discharge development occurring in the small gap between the contact charging member and the member to be charged.
In the case of the electrical discharge based charging mechanism there is a threshold voltage which must be surpassed by the charge bias applied to a contact type charging member before electrical discharge occurs between a contact type charging member and a member to be charged, and therefore, in order for the member to be charged through the electrical discharge based charging mechanism, it is necessary to apply to the contact type charging member a voltage with a value greater than the value of the potential level to which the member is to be charged. Thus, in principle, when the electrical discharge based charging mechanism is in action, the discharge product is unavoidable, that is, active ions such as ozone ions are produced, even though the amount thereof is remarkably small.
For example, a charging type using the electroconductive roller(charging roller) as the contact charging member is widely used because of its charging stability, but the discharge-using charging mechanism is dominant in the roller charging.
The charging roller comprises a rubber material or foam member having an electroconductivity or an intermediate resistance. In some charge rollers, the rubber or foamed material is layered to obtain a specific characteristic.
In order to maintain stable contact between a charge roller and a member to be charged, a charge roller is given elasticity, which in turn increases frictional resistance between the charge roller and the photosensitive member. Also in many cases, a charge roller is rotated by the rotation of a photosensitive drum, or is individually driven at a speed slightly different from that of the photosensitive drum. Therefore, occurrence of local non-contact state is unavoidable due to the non-uniformity of the roller shape and the deposited matter on the member to be charged. For this reason, discharge-using charging mechanism is dominant in the conventional charging mechanism using the roller charging. More particularly, when a charging roller is press-contacted to an OPC photosensitive member having a thickness of 25 xcexcm (member to be charged) to effect charging thereof, the charging roller is supplied with a voltage of not less than approx. 640 V, by which the surface potential of the photosensitive member starts to rise, and thereafter, with increase of the applied voltage, the surface potential of the photosensitive member rises linearly with inclination 1. Such a starting threshold voltage is called charge starting voltage Vth (broken line in FIG. 11).
In other words, in order to charge the surface of a photosensitive member to a potential level of Vd which is necessary for electrophotography, a DC voltage of (Vd+Vth), which is higher than the voltage level to which the photosensitive member is to be charged, is necessary. Hereinafter, the above described charging method in which only DC voltage is applied to a contact type charging member to charge a member will be called xe2x80x9cDC charging methodxe2x80x9d.
As for a counter measure for the above described problem, Japanese Laid-Open Patent Application No. 149, 669/1988 discloses an invention which deals with the above problem to effect more uniform charging of a photosensitive member. According to this invention, a xe2x80x9cAC charging methodxe2x80x9d is employed, in which a compound voltage composed of a DC component equivalent to a desired potential level Vd, and an AC component with a peak-to-peak Voltage which is twice the threshold voltage Vth, is applied to a contact type charging member. This is intended to utilize the averaging effect of alternating current. That is, the potential of the image bearing member to be charged is caused to converge to the Vd, that is, the center of the peaks of the AC voltage, without being affected by external factors such as operational ambience.
However, even in the case of the contact type charging apparatus in the above described invention, the principal charging mechanism is a charging mechanism which uses electrical discharge from a contact type charging member to the image bearing member. Therefore, as already described, the voltage applied to the contact type charging member needs to have a voltage level higher than the surface potential plus the discharge threshold. Thus, ozone is generated, although only in a small amount.
Further, when AC current is used so that member is uniformly charged due to the averaging effect of AC current, the problems related to AC voltage become more conspicuous. For example, more ozone is generated; noises traceable to the vibration of the contact type charging member and the photosensitive drum caused by the electric field of AC voltage increase; the deterioration of the photosensitive member surface caused by electrical discharge increases, which add to the prior problems.
(2) (2) Direct Charge Injection Mechanism
This is a mechanism in which the surface of a member to be charged is charged as electrical charge is directly injected into the member to be charged, with the use of a contact type charging member. This is proposed in Japanese Laid-open Patent Application No. HEI- 6-3921 or the like.
A contact type charging member with medium electrical resistance is placed in contact with the surface of a member to be charged to directly inject electrical charge into the surface portion of a member to be charged, without relying on electrical discharge, in other words, without using electrical discharge in principle. Therefore, even if the value of the voltage applied to a contact type charging member is below the discharge starting voltage value, the member to be charged can be charged to a voltage level which is substantially the same as the level of the voltage applied to the contact type charging member (solid line in FIG. 11 ) This direct injection charging mechanism does not suffer from the problems caused by the by-product of electrical discharge since it is not accompanied by ozone production.
More particularly, a voltage is applied to the contact charging member such as a charging roller, a charging brush or a charging magnetic brush to inject the charge to the charge retaining member such as a charge injection layer, electroconductive particles or the like or the trap level of the surface of the member to be charged(image bearing member) (direct injection charging). Since the discharge phenomenon is not dominant in this type, the voltage required for the charging is only the desired surface potential of the image bearing member, and therefore, there is no ozone production.
When the contact member comprises a porous roller such as a sponge roller and electroconductive particles thereon for improving the contact charging property, the contact between the contact charging member and the member to be charged can be made very close, so that satisfactory charging property can be provided.
Toner recycling process (cleanerless system)
In a transfer type image forming apparatus, the toner which remains on the peripheral surface of a photosensitive member (image bearing member) after image transfer is removed by a cleaner (cleaning apparatus) and becomes waste toner. Not only for obvious reasons, but also for environmental protection, it is desirable that waste toner is not produced. Thus, image forming apparatuses capable of recycling toner have been developed. In such an image forming apparatus, a cleaner is eliminated, and the toner which remains on the photosensitive member after image transfer is removed from the photosensitive drum by a developing apparatus; the residual toner on the photosensitive member is recovered by a developing apparatus at the same time as a latent image on the photosensitive drum is developed by the developing apparatus, and then is reused for development.
More specifically, the toner which remains on a photosensitive member after image transfer is recovered by tog removal bias (voltage level difference Vback between the level of the DC voltage applied to a developing apparatus and the level of the surface potential of a photosensitive member) during the following image transfer.
According to this cleaning method, the residual toner is recovered by the developing apparatus and is used for the following image development and thereafter; the waste toner is eliminated. Therefore, the labor spent for maintenance is reduced. Further, being cleanerless is quite advantageous in terms of space, allowing image forming apparatuses to be substantially reduced in size.
Japanese Laid-Open Patent Application No. 103, 878/1991 discloses a contact type charging apparatus with such a structure that coats a contact type charging member with electrically conductive powder, on the surface which comes in contact with the surface of a member to be charged, so that surface of the member to be charged is uniformly charged, that is, without irregularity in charge. The contact type charging member in this charging apparatus is rotated by the rotation of the member to be charged (without peripheral speed difference), and the amount of ozone generated by this charging apparatus is remarkably small compared to the amount of ozonic products generated by a corona type charging apparatus such as scorotron. However, even in the case of this charging apparatus, the principle, based on which a member is charged, is the same as the principle, based on which a member is charged by the aforementioned charge roller; in other words, a member is charged by electrical discharge. Further, also in the case of this charging apparatus, in order to assure that member to be charged is uniformly charged, compound voltage composed of DC component and AC component is applied to the contact type charging member, and therefore, the amount of ozonic products traceable to electrical discharge becomes relatively large. Thus, even this contact type charging apparatus is liable to cause problems; for example, images are affected by ozonic products, appearing as if flowing, when this charging apparatus is used for an extended period of time, in particular, when this charging apparatus is used in a cleanerless image forming apparatus for an extended period of time.
U.S. Pat. No. 5,432,037 discloses an image forming method using a contact charging wherein in order to avoid the charging problem due to deposition of the fine silica particles or toner particles during repeated long term image formation on the surface of the charging means, the developer contains at least visualizing particles and electroconductive particles having an average particle size smaller than that of the visualizing particles. However, the contact charging is based on the discharge-charging mechanism rather than the direct injection charging mechanism, and therefore, involves the above-described problems attributable to the discharging.
As described in the preceding paragraphs regarding the technologies prior to the present invention, it is difficult to directly charge a member with the use of a contact type charging apparatus with a simple structure which comprises a contact type charging member such as a charge roller or a fiber brush. Also in the case of an image forming apparatus which employs such a charging apparatus, the photosensitive member is liable to be insufficiently charged, causing images to appear foggy (during reversal development, toner is adhered to the areas which are supposed to remain white), or the photosensitive member is liable to be non-uniformly charged, causing image to be appear irregular in terms of continuity.
In the case of the contact type charging apparatus structured so that contact type charging member is rotated by the rotation of the photosensitive member, and so that photosensitive member is mainly charged by electrical discharge, ozonic products are liable to be accumulated, and images are affected by the accumulated ozonic products, appearing as if flowing, when such a charging apparatus is used for an extended period of time, in particular, when such a charging apparatus is used in a cleanerless image forming apparatus for an extended period of time.
Further, particularly in the case of the cleanerless image forming apparatus, there is the problem that residual toner causes the photosensitive member to be unsatisfactorily charged in a charging portion.
Accordingly, it is a principal object of the present invention to provide a charging device and an image forming apparatus wherein the contact property between the charging member and the member to be charged is improved even with the use of a simple member as a charging member.
It is another object of the present invention to provide a charging device and an image forming apparatus wherein the applied voltage to the charging member can be lowered so that substantially ozoneless charging accomplished.
It is a further object of the present invention to provide a charging device and an image forming apparatus wherein defect due to ozone product is avoided.
It is a further object of the present invention to provide a charging device and an image forming apparatus wherein infection charging is effected in a nip formed between a charging member and a member to be charged at low cost.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.